Here are
some of the best
maths books ever written.
Many more could be added to the list...Please do.

The Elements by Euclid
The foundations of geometry. Interestingly there is
now some doubt over whether Euclid wrote this book alone or whether
he was the leader of a team of mathematicians. Some even doubt that
he was a real historical figure.

Philosophiae naturalis principia mathematica by Isaac Newton
The Principia is one of the greatest scientific works ever.
It made startling advances in planetary motion and the theory of gravity.

Disquisitiones Arithmeticae by Carl Friedrich Gauss
Important work in number theory (although it also contains a ruler and compass
construction of a regular 17gon, the first progress in this field since
the ancient Greeks).
Éléments de Géométrie Algébrique
by Alexander Grothendieck
Completely rewrote alebraic geometry from the beginning. An enormously
influential book.

Atlas of Finite Groups by John H. Conway, Robert T. Curtis, Simon P. Norton, Richard A. Parker and Robert A. Wilson
The character tables in this book represent the main ingredient in, and a
summary of, the classification of finite simple groups.
It is the ultimate coffee table book because it so big you can put your
coffee cups on it!